The viscosity of a solution containing a given concentration of nitrocellulose depends on the chemical type of the solvent i.e. whether esters or ketones etc. within a particular chemical type, the solvent with the lowest boiling point give the lowest viscosity. As an example ketones of same boiling point as esters would give lower viscosity.

In mixed solvents, moderate amount of alcohols reduce viscosity. Minor amounts of hydrocarbon diluents have little effect, but as the proportion of hydrocarbon non solvent increases, the viscosity rises until finally the nitrocellulose becomes insoluble.

Ageing Effects

The viscosity of nitrocellulose solutions drops somewhat on storage. This fall is marked during first week and varies according to composition and storage temperature. The effect is attributable entirely to solvation and implies no chemical degradation of the nitrocellulose. There is no known way of preventing it and users are not recommended to try additives as a short cut to attaining equilibrium.

Film Properties

As with all polymers, the strength and durability of nitrocellulose lacquer films depends on the molecular weight, which in turn varies with viscosity. Thus the lowest viscosity grades e.g. HX 8/13 which have low molecular weight would the weakest, least durable film. These are recommended for high solid lacquers and inks where high gloss and build up are more important than durability and high film strength.

The latter properties improve as we move up the viscosity scale and HX30/50 represents the best compromise between good durability and reasonable solids. Where more flexibility is needed as in leather lacquers, HL 25/45 is recommended. Grades with viscosity higher than this offer no significant improvement in film properties and are used only in specialized outlets e.g. where a minimum of film is required or for blending with lower viscosity grades to make lacquers with a particular combination of solids and viscosity.

Stability

All industrial Nitrocellulose supplied passes the Bergmann and Junk stability test. However the following points should be noted:

The addition of alkaline and strongly acidic material is harmful

Certain amines (diethylamine, monoethanolamine, morpholine etc.) if allowed to come in contact with industrial Nitrocellulose can cause spontaneous ignition or charring. There is a risk of fire particularly with dry nitrocellulose. Any spillages should be cleaned up immediately.

These amines also cause severe discolouration of nitrocellulose films and solutions. The use of alcohol denatured with pyridine should be rigorously avoided.
Avoid storing nitrocellulose or nitrocellulose lacquers at high temperature or in direct sunlight.
Avoid using pigments which give an alkaline reaction.

Blending

With the wide range of viscosity grades supplied, it is not normally necessary to blend different grades of nitrocellulose. Occasionally, however, stocks of a particular grade may be exhausted, or there may be a special requirement that cannot be met by available materials. In such cases mixtures of suitable grades will give the desired viscosity.

Blending Limitations

It is not advisable to blend different nitrogen grades or any grades which are far apart in viscosity, since either procedures can cause rough, granular solutions or resin incompatibility problems.